Electroactive materials can be broadly separated into three types of materials: piezoelectric materials, elastomers between two electric plates, and ion-containing materials. Most piezoelectric materials undergo length changes of only a fraction of one percent. The movement from electroactive materials that use an elastomer between two electric plates is visible to the naked eye, however these materials use extremely high voltages, measured in the kilovolt range, and once that type of electroactive material is activated it remains static. With ion-containing electroactive materials, the material itself responds to electricity by movement that is visible to the naked eye, and as long as the electricity is on, these materials typically continue to move. The voltage requirements for ion-containing electroactive materials are much lower than elastomeric electroactive materials, typically less than 100 volts. Historically, ion-containing electroactive materials have had some drawbacks: many of the ion-containing electroactive polymers are inherently weak materials and, typically being hydrogels, if they dry out, then they become hard, brittle, inflexible, and thus electrically unresponsive.
Applicant has previously found that copolymers comprising cross-linked networks of methacrylic acid and 2-hydroxyethyl methacrylate, (PMA-co-PHEMA) cross-linked with cross-linking agents such as ethylene glycol dimethacrylate and 1,1,1-trimethylolpropane trimethacrylate, are superior ionic electroactive materials, with tensile strengths well above the tensile strengths of other ion-containing electroactive materials found in the prior art at that time (U.S. Pat. No. 5,736,590, [1998]). A relatively small amount of electricity caused movement.
TABLE 1Strengths of some common ion-containing electroactive polymerscompared to PMA-co-PHEMA cross-linked networks:MaterialsTensile Strength (MPa)Poly(acrylamide) gels0.03Poly(vinyl alcohol)-poly(acrylic acid) gels0.23Poly(hydroxyethyl meth acrylic acid)-poly0.33(methacrylic acid) cross-linkednetwork gels††0.28 to 0.76 MPa range for these types of materials
In 2004 and 2005, applicant developed strong electroactive materials that had pronounced responsive movement to electricity, which led to another drawback. If the electroactive material responded quickly with a lot of movement, then the electrodes often detached. If even one electrode detached, then the actuator failed. This challenge was addressed by plasma treating the electrodes to improve the polymer-metal interface, so that the electrodes and the electroactive material would work as a unit, similar to how nerves are integrated into muscle tissue. By plasma treating the electrodes, which are inserted or embedded into the electroactive material, a much better polymer-metal interface could be achieved between the embedded electrodes and the electroactive material as described in applicant's U.S. patent application Ser. No. 11/478,431. A good polymer-metal interface is crucial because the electroactive materials developed by applicant undergo pronounced movement. Applicant has found that by encapsulating or coating the electroactive materials, with embedded electrodes, the actuator can be free-standing, independent of submersion in an electrolytic solution as described by applicant's U.S. patent application Ser. No. 11/478,431.
The novel electroactive materials and electroactive actuators described in the instant patent application use ion-containing electroactive materials that are produced within a defined range of cross-linking, along with other considerations, such as dilution of the monomer mix, choice of electrolyte, and the configuration of the electrodes, which allow for the preferred movement of contraction. Electroactive polymers in the prior art undergo a variety of movement. The movement of contraction is considered to be an extremely useful movement because of the similarity to movement produced by muscle tissue. The instant patent application discloses compositions of electroactive materials that undergo contraction and electrode configurations that further increase contraction in these electroactive materials and electroactive actuators. The instant patent application also discloses a novel, superior method to significantly improve the polymer-metal interface, preferably by plasma treating the titanium metal electrodes of the actuators with nitrogen plasma, followed by oxygen plasma or treated individually with either nitrogen plasma or oxygen plasma. By encapsulating or coating the electroactive materials, with embedded electrodes, these actuators can be operational anywhere.